The present invention relates to a liquid crystal display (LCD) controller.
An example of a liquid crystal display controller is disclosed by JP-A-No. 11-311980. The controller can drive part of the liquid crystal display selectively as well as can set the operating voltage, operation bias, and reference clock frequency according to the number of active lines in the selected part. This means that when the entire screen need not be displayed, only the required part of the screen can be displayed under proper operating conditions. This leads to reduction in electric power consumption.
In the prior art, the frame frequency is held constant by varying the ratio of division of the original clock depending on the number of active lines and using the resulting clock as the reference clock.
Assume that 16 lines are activated for a partial display in a liquid crystal panel, for example, where on the basis of eight lines per row, the whole screen has 32 lines (4 rows). In this case, if the reference clock frequency is constant, the frame frequency doubles since the time for activating 16 lines is half that for 32 lines. As a result, the image quality may deteriorate, for example, shadowing may occur. To avoid this, the original clock is halved or divided into two clocks and the resulting clock is used as the reference clock to hold the frame frequency constant. Likewise, by dividing the original clock into four or eight clocks, the frame frequency can be held constant for a partial display of eight lines (two rows) or four lines (one row).
Recently there is an increasing tendency for liquid crystal panels in cellular phones or similar devices to use more than 100 display lines. Also there is demand for the number of lines per row to be other than 8, such as for a partial display. However, the number of lines for a partial display to hold the frame frequency constant is limited to ½, ¼, ⅛ and so on of that for the full screen display; therefore it is difficult to hold the frame frequency constant without limitations on the number of active lines.
The optimum frame frequency may differ depending on the characteristics of the liquid crystal panel. For example, if a liquid crystal with quick brightness response is used, generally the frame frequency must be increased to obtain a satisfactory contrast, though a higher frame frequency leads to an increase in power consumption. One solution is to make the frame frequency variable depending on the application purpose, for example, by preparing two modes: a contrast-oriented mode and a power saving mode. However, conventional liquid crystal display controllers are not designed to change the frame frequency depending on the operating mode of the liquid crystal display unit.
For cellular phones with liquid crystal displays, mobile communication terminals with scheduling functions and other similar devices, there is growing demand for a model which can be used for an extended time with less power consumption. In the standby or waiting state of a cellular phone or when a mobile communication terminal is in operation, there is a need to display not the whole screen but, for example, part of the screen needed for the clock.
Thus there is a need for a liquid crystal display controller that can provide a full/partial display with good display quality and/or low power consumption. In addition there is a need for both keeping the frame frequency constant and for varying it under a wider range of conditions.